Magnetic field sensors (e.g., rotation detectors) for detecting ferromagnetic articles and/or magnetic articles are known. The magnetic field associated with the ferromagnetic article or magnetic article is detected by a magnetic field sensing element, such as a Hall element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal) proportional to a detected magnetic field. In some arrangements, the magnetic field signal is an electrical signal.
The magnetic field sensor processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses thresholds, either near to peaks (positive and/or negative peaks) or near to some other level, for example, zero crossings of the magnetic field signal. Therefore, the output signal has an edge rate or period indicative of a speed of rotation of the ferromagnetic or magnetic object, for example, a gear or a ring magnet.
One application for a magnetic field sensor is to detect the approach and retreat of each tooth of a rotating ferromagnetic gear, either a hard magnetic gear or a soft ferromagnetic gear. In some particular arrangements, a ring magnet having magnetic regions (permanent or hard magnetic material) with alternating polarity is coupled to the ferromagnetic gear or is used by itself and the magnetic field sensor is responsive to approach and retreat of the magnetic regions of the ring magnet. In other arrangements, a gear is disposed proximate to a stationary magnet and the magnetic field sensor is responsive to perturbations of a magnetic field as the gear rotates.
In one type of magnetic field sensor, sometimes referred to as a peak-to-peak percentage detector (or threshold detector), one or more threshold levels are equal to respective percentages of the peak-to-peak magnetic field signal. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold” and assigned to the assignee of the present invention.
Another type of magnetic field sensor, sometimes referred to as a slope-activated detector (or peak-referenced detector, or peak detector for short), is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” also assigned to the assignee of the present invention. In the peak-referenced magnetic field sensor, the threshold signal differs from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of magnetic field sensor, the output signal changes state when the magnetic field signal comes away from a peak or valley of the magnetic field signal by the predetermined amount.
It should be understood that, because the above-described threshold detector and the above-described peak detector both have circuitry that can identify the positive and negative peaks of a magnetic field signal, the threshold detector and the peak detector both include a circuit portion referred to as a “peak identifier” herein, which is configured to detect positive peaks and/or negative peaks of the magnetic field signal. The threshold detector and the peak detector, however, each use the detected peaks in different ways to provide a so-called “threshold generator,” which is configured to use the identified peaks to generate one or more thresholds against which the magnetic field signal can be compared. This comparison can result in a so-called “PosComp” signal that has an edge rate representative of a speed of movement, e.g., rotation, of the moving object.
In order to accurately detect the positive and negative peaks of a magnetic field signal, in some embodiments, the rotation detector can be capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
Some types of rotation detectors perform one or more types of initialization or calibration, for example, at a time near to start up or power up of the rotation detector, or otherwise, from time to time as desired. During one type of calibration, the above-described threshold level is determined.
Once the above-described threshold level is initially determined, various schemes may be used for updating the threshold level to ensure that the threshold level remains at the desired relationship with respect to the peak-to-peak magnetic field signal level. For example, as described in U.S. Pat. No. 6,525,531 entitled “Detection of Passing Magnetic Articles while Adapting the Detection Threshold” and assigned to the assignee of the subject invention, the positive and negative detected peak signals (PDAC and NDAC, respectively) freely track “outwardly” to follow the magnetic field signal as it increases above PDAC and decreases below NDAC, respectively, following which such detected peak signals are selectively allowed to move “inward” (i.e., PDAC decreases and NDAC increases) to the level of the magnetic field signal upon transitions of the PosComp signal. Such threshold signal updating may be performed following an initial calibration mode, such as during a “running mode” of operation.
Many types of magnetic field sensors do not provide an accurate output signal (e.g., indication of absolute angle, speed, or direction of rotation) immediately upon power up, upon movement of the target object from zero rotating speed, and/or upon movement slowing to zero rotating speed, but instead provide an accurate output signal only once the target object has moved through a substantial rotation or is moving with substantial speed. For example, in one type of magnetic field sensor described in U.S. Pat. No. 6,525,531, entitled “Detection of Passing Magnetic Articles while Adapting the Detection Threshold,” issued Feb. 25, 2003, a positive digital-to-analog converter (PDAC) and a negative digital-to-analog converter (NDAC) track positive and negative peaks of a magnetic field signal, respectively, for use in generating a threshold signal. A varying magnetic field signal (an electrical signal responsive to a magnetic field) is compared to the threshold signal. However, the outputs of the PDAC and the NDAC may not be accurate indications of the positive and negative peaks of the magnetic field signal until several cycles of the signal (i.e., signal peaks) occur (i.e., until several gear teeth have passed). This type of magnetic field sensor, which generally requires time to become fully accurate, is referred to herein as a so-called “precision rotation detector.”
In contrast, a “true power on state” (TPOS) detector can provide a reasonably accurate output signal shortly after movement of a target object (e.g., camshaft) from zero rotating speed, or a low rotation speed in some applications of for example less than 100 rpm, or also shortly before movement slowing to zero rotating speed. Furthermore, even when the target object is not moving, the TPOS detector can provide an indication of whether the TPOS detector is in front of a tooth or a valley of a gear. However, when the target object is stationary, the conventional TPOS detector is not able to identify an absolute or relative angle of rotation of the target object. The TPOS detector can be used in conjunction with a precision rotation detector within a common integrated circuit, each providing information to the engine control processor at different times. For simplicity, TPOS detectors and precision rotation detectors are shown herein within a common integrated circuit. However, the TPOS detector or the precision rotation detector can also be used alone in separate circuits.
The TPOS detector can be a so-called “tooth detector,” able to differentiate a tooth from a valley when the gear is not rotation. The precision rotation detector can be a so-called “edge detector,” able to detect edges of gear teeth as they pass by, or the precision rotation detector can be a tooth-detector.
Some moving objects, for example, rotating moving objects, which are sensed by the above-described magnetic field sensors, exhibit irregular motions or have irregular features. For example, a gear may have wobble as it rotates, it may have run out (asymmetry about its axis of rotation), or it may have irregularities in its mechanical dimensions, for example, some gear teeth may be wider than others. These irregularities tend to result in generation of thresholds that are not ideal. The non-ideal threshold tends to result in a PosComp signal that has edges that are not accurately placed relative to cycles of the magnetic field signal associated with the moving object.
It would, therefore, be desirable to provide a magnetic field sensor that can accurately identify threshold levels associated with a magnetic field signal, accurate even in the presence of irregularities in the motion of, or in the mechanical characteristics of, the moving object being sensed. It would also be desirable to provide a magnetic field sensor that can accurately identify threshold level for a magnetic field signal associated with a rotating object having a number of detectable features selected from a set of possible numbers of detectable features.